The potential of hydrogen as an environmentally-friendly fuel source has been studied extensively worldwide. As a fuel for vehicles, hydrogen is capable of dramatically reducing harmful emissions, as compared to the combustion of carbon-based fossil fuels. Hydrogen can also serve as the feedstock for fuel cells, or it can be burned in combustion engines in a manner comparable to that for gasoline in existing internal combustion engines. Furthermore, in contrast to gasoline or other hydrocarbon fuels, the primary byproduct of burning hydrogen in oxygen or air is water. Similarly, no air pollutants or greenhouse gases are produced when hydrogen is used in fuel cells.
Development of the so-called “hydrogen economy” has the potential to weaken or completely eliminate the current reliance on methane or gasoline. In 2008, Muradov and Veziroglu advocated the production of “carbon-neutral synthetic fuels from bio-carbon and hydrogen generated from water . . . ”. Muradov, N. Z. and Veziroglu, T. N., “Green' Path from Fossil-Based to Hydrogen Economy: An Overview of Carbon-Neutral Technologies,” International Journal of Hydrogen Energy, 33, 6804-6839 (2008).
The primary impediment to the development of such an economy is the introduction of efficient methods for producing hydrogen, and preferably doing so by point-of-use methods that do not require hydrocarbons as feedstocks to the process. At present, the most prevalent means for producing hydrogen are steam natural gas (methane) reforming and electrolysis. The former entails producing synthetic gas, a mixture of hydrogen, carbon monoxide, and carbon dioxide, by reacting natural gas with steam. Approximately 96% of the hydrogen generated worldwide is synthesized by this process (S. V. T. Nguyen, J. E. Foster, and A. D. Gallimore, Rev. Sci. Instrum., vol. 80, 083503 (2009)). Electrolysis is the process in which water is dissociated into oxygen and hydrogen by passing an electrical current through liquid water. Although electrolysis is capable of achieving efficiencies of 70-75%, this process requires high current densities, and the presence of considerable quantities of water in the reaction vessel. Therefore, the electrolysis of water generally requires the proximity of a substantial power source and, for this reason, several of the largest electrolysis sources of hydrogen are located near hydroelectric generating stations. Only 4% of the hydrogen produced worldwide is generated by electrolysis at present.
Much of the hydrogen produced in the United States is currently used for refining petroleum, treating metals, producing fertilizer, and processing foods. In order to make hydrogen more competitive as a fuel, the cost for its production must be reduced. Furthermore, the growing interest in transitioning the world's economies from a dependence on carbon-based fuels to renewable sources of energy suggests that a hydrogen production process that is not dependent on hydrocarbon feedstocks is desirable. As a result, researchers have been investigating the dissociation of water by several techniques. In one approach, high temperatures generated by solar concentrators or nuclear reactors drive chemical reactions that dissociate water to produce hydrogen. In another approach, microbes, such as green algae, consume water in the presence of sunlight, producing hydrogen as a byproduct. There are also photo-electrochemical systems that produce hydrogen from water using particular semiconductors and energy from sunlight.
Researchers have also investigated using plasmachemical reactions to dissociate water. Such processes have several advantages, as compared to the above-discussed methods, including the potential for product specificity by tailoring the characteristics of the plasma (electron temperature, electron density, etc.) and scalability in volume.
Jung et al. published a study of water dissociation in a microwave plasma (Jung et al., “Hydrogen Generation from the Dissociation of Water Using Microwave Plasmas,” Chin. Phys. Letters, Vol. 30, No. 6 (2013). These experiments involved the dissociation of water vapor at pressures kept below 10-50 Torr, and the authors state that “it is difficult for water to be split (direct dissociation) by atmospheric pressure thermal plasma since the high electron collision rate . . . causes a very strong decrease in electron temperature.” The electron densities were on the order of 1012 cm−3 and the measured rate of hydrogen production was 1.8-2.7 grams of hydrogen per kWh of input electrical power to the reactor.
Nguyen et. al. have published results concerning water dissociation using an RF plasma generator. Nguyen et. al., “Operating a radio-frequency plasma source on water vapor,” Review of Scientific Instruments 80, 083503 (2009). In this work, water vapor having a pressure of 300 mTorr was partially dissociated in an RF discharge driven at a frequency of 13.56 MHz. The reactor vessel was a quartz tube having a diameter of 15 cm and a length of 50 cm. The authors reported production of approximately 20 sccm of H2 for 500 W of RF power.
Chen et al., “H2O splitting in tubular PACT (plasma and catalyst integrated technologies) reactors,” J. Catal., vol. 201, no. 2, pp. 198-205, (July 2001) used tubular plasma and catalyst reactors with gold as a catalyst to dissociate water vapor carried by a stream of Ar. They achieved a dissociation degree of 14% and the highest energy efficiency realized was approximately 2%. In a study conducted by Suib et al., an energy efficiency of 1.1% was achieved without adopting a Pt catalyst. (S. L. Suib, Y. Hayashi, and H. Matsumoto, “Water splitting in low-temperature AC plasmas at atmospheric pressure,” Res. Chem. Intermed., vol. 26, no. 9, pp. 849-874, (2000)). Kabashima et al. used a packed-bed plasma reactor to dissociate a stream of water vapor entrained in N2. (H. Kabashima, et al., “Hydrogen generation from water with nonthermal plasma,” Chem. Lett., vol. 30, no. 12, pp. 1314-1315 (2001)). They were able to demonstrate 63% of water dissociation but at an energy efficiency of less than 1%.